Krasnikov tube vs Worm Holes and Alubierre Drive

[viperzerofsx]

Hello!

So i've been working on a sci fi project for a while and one goal is for my project to be as realistic as possible in terms of science and yet still allow some way to get around the universe in a plot workable time frame. Anyway I've been looking into the different methods of plausible faster then light travel. I'm no Physicist so everything is really based on a "that seems reasonable" sort of way of thinking.  Everyone tells me the Alcubierre drive isn't really likely to be workable, I understand that. Wormholes seem to be a bit more iffy and thats something i'm looking into. However one think i've looked into that seems to make sense is a Krasnikov Tube. The idea is sort of like this. I leave Earth at relativistic speeds in 2017 and head for Alpha Centauri in late 2021 early 2022, what ever. I leave a Krasnikov tube in my wake. I see the sights get to know the locals take some pictures go home. Now I follow my Tube and reach earth right after I left in 2017. To me, thanks to Time dilatation going to Alpha Centauri and the tube on my way home, It seems to me that I went faster then light, but I never did, and the tube let me go back home right after I left. 

I like this idea because its almost a poor man's warp drive and it seems less problematic.  You can also still also have fun building realistic relativistic spaceships. I don't know if it really is any more plausible then say a wormhole but if it is it might be worth looking into.

[AmrlKJaneway]

There are papers that show that two Krasnikov Tubes arranged in a certain way can lead to time travel paradoxes.

Myself, I prefer to go with no FTL at all. With relativity, longer lifespans, multiple copies of individuals, and deep sleeps, most of the plot reasons that require FTL can be worked around when you consider future society.

But if you HAVE to use it, then OA's wormholes are probably the best solution. The idea that they destabilize if accelerated to relativistic velocities means they cannot be arranged as time machines, so you get all the benefits of zipping around the galaxy, without the headache of time travel.

[Alphadon]

There are papers that show that two Krasnikov Tubes arranged in a certain way can lead to time travel paradoxes.

Myself, I prefer to go with no FTL at all. With relativity, longer lifespans, multiple copies of individuals, and deep sleeps, most of the plot reasons that require FTL can be worked around when you consider future society.

But if you HAVE to use it, then OA's wormholes are probably the best solution. The idea that they destabilize if accelerated to relativistic velocities means they cannot be arranged as time machines, so you get all the benefits of zipping around the galaxy, without the headache of time travel.

Small nitpick: You can accelerate a wormhole mouth to relativistic speeds, but if you put them close enough together to create a time machine, then they destabilize.
Come to think of it, such a concept of chronology protection would probably destroy Krasnikov tubes only if they are in a time-travel configuration, making them possible.

[Rynn]

Small nitpick: You can accelerate a wormhole mouth to relativistic speeds, but if you put them close enough together to create a time machine, then they destabilize.
Come to think of it, such a concept of chronology protection would probably destroy Krasnikov tubes only if they are in a time-travel configuration, making them possible.

This is why the Nexus grows outwards, wormholes travelling at relativistic speeds shift into the future from the perspective of the wormhole that stays at home. That doesn't violate causality because even if home (Hw) is in the global past relative to colony (Cw) it's outside of Cw's light cone. A krasnikov tube would have the same issue I'd think, though I don't know much about them.

[AmrlKJaneway]

So, would an Alcubierre metric traveling faster than light also only collapse if it was being steered in a direction where it could run into itself?

edit; my bad on the mistake. I was positive I read somewhere that if high enough relativistic velocities were achieved, Lorentz contractions would alter the size of the wormhole mouth and destabilize it...

edit 2; this is what I'm thinking of - "2) Linear instabilities: Wormholes are typically subjected to linear instabilities during the deployment phase, right after the wormhole has been inflated from the quantum regime, but before it is inflated to traversable size. These instabilities come from Lorentz contraction during the transport of the wormhole mouths to their final destinations and are typically limited to perturbations of less than 50% of the wormhole rest mass. For this reason, wormhole transport velocities are constrained to less than .74c." Although it goes on to say that this restriction doesn't matter if the mouth is transported within a void bubble.

[Alphadon]

So, would an Alcubierre metric traveling faster than light also only collapse if it was being steered in a direction where it could run into itself?

edit; my bad on the mistake. I was positive I read somewhere that if high enough relativistic velocities were achieved, Lorentz contractions would alter the size of the wormhole mouth and destabilize it...

edit 2; this is what I'm thinking of - "2) Linear instabilities: Wormholes are typically subjected to linear instabilities during the deployment phase, right after the wormhole has been inflated from the quantum regime, but before it is inflated to traversable size. These instabilities come from Lorentz contraction during the transport of the wormhole mouths to their final destinations and are typically limited to perturbations of less than 50% of the wormhole rest mass. For this reason, wormhole transport velocities are constrained to less than .74c." Although it goes on to say that this restriction doesn't matter if the mouth is transported within a void bubble.

Although this is true, this is due to the nature of the wormhole itself and is not the same as chronology protection, which takes effect only in the event of closed timelike curves.

[Rynn]

It's been a while since I've read up on Kraznikov tubes. Bringing up the wiki article has a succinct explanation of how two tubes can be used to make a time machine, just like wormholes. Matt Visser's hypothesis that gravity fluctuations would lead to a collapse at the moment of Closed Timelike Curve creation is, again, just like the wormholes OA uses.

While one Krasnikov tube can be seen to present no problems with causality, it was proposed by Allen E. Everett and Thomas A. Roman of Tufts University that two Krasnikov tubes going in opposite directions can create a closed timelike curve, which would violate causality.[3]

For example, suppose that a tube is built connecting Earth to a star 3000 light years away. The astronauts are travelling at relativistic velocities, so that the journey through this Tube I only takes 1.5 years from their perspective. Then the astronauts lay down tube II rather than travelling back in tube I, the first tube they produced. In another 1.5 years of ship time they will arrive back on Earth, but at a time 6000 years in the future of their departure. But now that two Krasnikov tubes are in place, astronauts from the future can travel to the star in tube II, then to Earth in tube I and will arrive 6000 years earlier than their departure. The Krasnikov tube system has become a time machine.

In 1993, Matt Visser argued that the two mouths of a wormhole with an induced clock difference could not be brought together without causing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other.[4] (See Time travel using wormholes and the Chronology protection conjecture.) It has been suggested that a similar mechanism would destroy time-machine Krasnikov tubes. That is, vacuum fluctuation would grow exponentially, eventually destroying the second Krasnikov tube as it approaches the timelike loop limit, in which causality is violated.

[Drashner1]

The bottom line with all of these forms of metric engineering (warp drives, wormholes, or Krasnikov tubes) is that they rely on the same basic technology - that is to say a method of generating and controlling significant to astronomical amounts of exotic matter/energy that can violate the Average Null Energy Condition (ANEC). And because space and time are one and the same thing (space-time) then any kind of messing around with space is going to result in some form of messing around with time - which means potential time machines and potentially some form of Visser Collapse or an equivalent.

Going back to the original post on the thread - I'm not sure that it's correct to speak in terms of one or another of these being more or less realistic or plausible or 'hard science' - because they are all essentially different sides of the same coin in many respects. You can differentiate based on what you're wanting to do (and the tradeoffs involved) and the amount of exotic mass-energy required or the stability of the thing you are creating. But I don't think it really makes sense to speak in terms of what is more or less 'hard science'

My 2c worth,

Todd

[stevebowers]

So, would an Alcubierre metric traveling faster than light also only collapse if it was being steered in a direction where it could run into itself?

Alcubierre metrics travelling faster than light would run into causality problems under a wide set of circumstances, since they could be used to send messages back in time, especially between observers who are travelling relativistically with respect to one another. I don't know if they would collapse, but in OA void bubbles (which are based on the Alcubierre metric) are limited to subliminal speeds so the problem doesn't arise.

[Drashner1]

So, would an Alcubierre metric traveling faster than light also only collapse if it was being steered in a direction where it could run into itself?

Alcubierre metrics travelling faster than light would run into causality problems under a wide set of circumstances, since they could be used to send messages back in time, especially between observers who are travelling relativistically with respect to one another. I don't know if they would collapse, but in OA void bubbles (which are based on the Alcubierre metric) are limited to subliminal speeds so the problem doesn't arise.

To expand on Steve's response a bit:

In OA, warp bubbles can (in principle) reach or exceed the speed of light - however, if they try, virtual particle creation inside the bubble ramps up exponentially until the bubble is destroyed. This is actually based on a RL theory referenced in one of the reference papers listed at the end of the article. For OA purposes we presume it to be true as a way to make use of warp bubbles without running into causality issues.

So it's not really a matter of where the warp is going as whether or not it reaches or exceeds the speed of light.

Todd